1. Field of the Invention:
This invention relates to remotely disposed automatically operated mechanism for securing heat exchanger tube ends within the tube sheet of a steam generator, and more particularly, to switch means on such apparatus for sensing the presence of the tube ends on the apparatus and the proper positioning of the tube ends within the tube sheet.
2. Description of the Prior Art:
Retubing of a steam generator generally includes a procedure for initially placing the opposite ends of the U-shaped heat exchanger tubes substantially flush with the lower face of a tube sheet and then securing the tube ends to the tube sheet in this position.
In a typical steam generator such operation on a single tube end can be done manually and proper location is visually obtained using indexing or locating shoulders formed as a part of the tool securing the tube ends within the tube sheet. However, in the environment of an irradiated nuclear steam generator the manual procedure for securing the tube ends in the tube sheet has been replaced by automatic apparatus remotely disposed within the nuclear steam generator head such as more fully described in previously identified, commonly owned, U.S. Pat. No. 4,178,787, entitled "Remotely Operated Tube Expanding Tool and Support". Thus, in such environment, each tube has opposed ends projecting downwardly from the tube sheet on either side of the lower head divider plate. The automatic apparatus includes a tool support mechanism remotely disposed on each side of the divider plate, with each mechanism capable of moving the projecting adjacent tube end upwardly into proper position within the tube sheet prior to the operation of securing the tubes via a tool on the mechanism. Preferably, each mechanism is associated with the opposite ends of a common U-shaped heat exchanger tube. However, it is apparent that if each mechanism is disposed equidistantly below the tube sheet in axial alignment with the tube ends, and then raised into abutting engagement with the tube sheet to position the tube ends therewithin, unless the tube ends likewise project an equidistance from the tube sheet, both ends (because of stiffness of the heat exchanger tube) could move upwardly a distance equivalent to the upward axial movement of the furthest projecting end. This would result in the lesser extending end of the common tube being disposed upwardly in the tube sheet generally inaccessible to the tool for securing the tube and tube sheet together. Thus, the end of the tube which projects the least from the tube sheet must be initially moved into the proper position in the tube sheet and at least temporarily secured thereto, and the opposite end which will still be projecting somewhat from the tube sheet subsequent to this operation subsequently moved into proper position with the upward movement of this end accommodated by the resiliency of the upper bend of the U-shaped tube while the secured end remains in proper position in the tube sheet. The above-operational procedure is under the control of a servicing system and comprises only a part of the complete servicing system for controlling the operation of the remote apparatus for retubing the generator. The complete system is more fully described in concurrently filed, co-owned application Ser. No. 952,431 entitled "Method and Apparatus for Servicing a Steam Generator".